1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission which is shiftable between different gear ratios and which has a plurality of hydraulically operated frictional elements for effecting shifting.
2. Prior Art
U.S. patent application Ser. No. 462,337, now U.S. Pat. No. 4,573,375 issued to Hideo Hamada et al. on Mar. 4, 1986, discloses a hydraulic control system for an automatic transmission. This hydraulic control system equipped with a lock-up control has been employed in an E4N71 type Automatic Transmission manufactured by Nissan Motor Company Limited, and it is described in "SERVICE MANUAL FOR NISSAN AUTOMATIC TRANSMISSION L4N71B TYPE AND E4N71B TYPE" issued by Nissan Motor Company Limited in November 1982 (see Page 38). For better understanding of this hydraulic control system, reference should be made to the above mentioned U. S. Pat. No. 4,573,375.
This known transmission is shiftable between different gear ratios and has a plurality of frictional elements including a rear clutch (a forward clutch), a front clutch (a high and reverse clutch), a second brake, a low and reverse brake, a direct clutch, and an OD (overdrive) band brake, to effect shifting. The hydraulic control system comprises an oil pump serving as a fluid source, a pressure regulator valve communicating with the fluid source for generating a servo actuating fluid pressure, often called a "line pressure," a plurality of shift valves, each having an inlet port communicating with the line pressure regulator and an outlet port communicating with the corresponding one/ones of the plurality of frictional elements for selectively supplying hydraulic fluid to such frictional element or discharging hydraulic fluid therefrom, a plurality of flow rate adjusting means, including a flow restrictor, an accumulator, and a timing valve, fluidly disposed between the frictional elements and the shift valves. Each of these flow rate adjusting means is so designed as to provide an optimum pattern of variation of pressure build-up at the frictional element to be engaged to effect shifting without any substantial shocks. Thus, the circuit of this hydraulic control system is not simple.
A task to be solved by the present invention is to improve the hydraulic control system such that, with a simple hydraulic circuit, the pattern of variation of pressure build-up at a frictional element to be operated to effect shifting is changeable in response to operating conditions and also environmental conditions where the shifting is effected in order to reduce substantial shocks which occur during the shifting. For example, the pattern of variation of pressure build-up differs from one situation where the shifting is effected at a less abrupt acceleration to another where the shifting is effected at an abrupt acceleration. The environment where the shift is effected involves a change in temperature of the hydraulic fluid.